Conventionally, a system as shown in FIG. 1 is known as the above-mentioned system.
FIG. 1 schematically shows the arrangement of a system that creates a page layout document of DTP or the like, or a wordprocessing or graphic document using a host computer 101, and outputs such document as a hardcopy via a color laser beam printer, ink-jet printer, or the like.
Reference numeral 102 denotes an application which runs on the host computer. For example, document edit application software such as “WORD®” available from Microsoft Corporation, page layout software such as PageMaker® available from Adobe Systems Incorporated, and the like are representative ones.
A digital document created by such software is passed on to a printer driver 103 via an operating system (OS: not shown) of the computer.
The digital document is expressed as a set of command data that represent figures, characters, and the like which form one page, and these commands are sent to the printer driver 103. The printer driver 103 converts a series of commands as a language system called PDL (page description language). As typical PDLs, GDI®, PS® (PostScript), and the like are prevalent.
The printer driver 103 transfers generated PDL command data to a rasterizer 105 in a raster image processor 104. The rasterizer 105 renders characters, figures, and the like expressed by PDL commands into a two-dimensional bitmap image that the printer actually outputs. This device is called a “rasterizer” since a bitmap image fills a two-dimensional plane as repetitions of linear rasters (lines). The rendered bitmap image is temporarily stored in an image memory 106.
FIG. 2A depicts the aforementioned operations. A document image 111 displayed on the host computer is sent as a PDL command sequence 112 to the rasterizer via the printer driver, and the rasterizer renders a two-dimensional bitmap image 113 on the image memory.
The rendered image data is sent to a color printer 107. The color printer 107 uses a known electrophotographic or ink-jet recording image forming unit 108, and prints out an image by forming a visible image on a paper sheet using such unit. The image data in the image memory is transferred in synchronism with a sync signal and clock signal (not shown) required for operating the image forming unit, a transfer request of a specific color component signal, or the like.
In the aforementioned prior art, upon examining an image forming unit used in output, various problems are posed.
For example, in order to form a color image on a printout medium, a color printer normally forms an image using four colors, i.e., cyan (C), magenta (M), yellow (Y), and black (K) toners or inks on the basis of so-called subtractive color mixing.
On the other hand, upon displaying an image, the application on the host computer normally uses a color monitor, which displays colors using additive primaries, i.e., red (R), green (G), and blue (B).
Hence, all of colors of characters, figures and figures that form a document, and colors of images laid out by scanning photos and the like via a scanner are expressed as those obtained by mixing R, G, and B.
That is, the rasterizer must convert color information which is defined by R, G, and B as PDL and transferred from the host computer into C, M, Y, and K by some means, and must then generate a bitmap image and output it to the printer.
However, a method of converting R, G, and B into C, M, Y, and K is not uniquely determined, and an optimal conversion method differs depending on the attributes of figures defined by PDL. For example, referring to FIG. 2A, reference numeral 114 denotes a natural image scanned by, e.g., a scanner; 115, graphic images such as a rectangle and the like, which are electronically generated; and 116, character (TEXT) images. That is, these images have different attributes.
When the color of each TEXT image 116 is defined as black (R=G=B=0), optimal C, M, Y, and K signals corresponding to this color are expressed by 8-bit density signals C=M=Y=0 and K=255. That is, a black character is preferably reproduced using only black toner of the four color toners of the printer.
On the other hand, when a specific pixel value of the natural image 114 is R=G=B=0, if that value is converted into C=M=Y=0 and K=255 in the same manner as the character data, the highest density portion in the natural image is reproduced using only black toner, and the absolute density becomes insufficient. In this case, a better result is obtained when this pixel value is converted into a value given by C=M=Y=100 and K=255 to increase the absolute density.
In order to solve such problem, the following method may be used. That is, the rasterizer renders input R, G, and B values into a bitmap image without converting them into C, M, Y, and K, and the image forming unit detects a character image area from the received RGB bitmap image using known image area separation, and uses different RGB to CMYK conversion methods for the detected character image area and other areas to generate and output CMYK data.
However, the image area separation cannot always perfectly detect a character area, and often detects some natural image area as a character area. Hence, an image is formed based on wrong detection results, thus disturbing improvement of the quality of an image to be recorded.
As another example, the image forming unit can express only binary dots. In such case, the rasterizer renders Y, M, C, and K multi-valued bitmap image data on the image memory, and the image forming unit that receives the bitmap data converts the multi-valued image signal into a binary image signal by a known binarization process such as error diffusion, dithering, or the like, and prints it out.
At this time, an optimal binarization method also differs depending on image attributes. That is, graphic data such as characters, figures, and the like preferably undergo binarization that uses a small matrix size of dithering and attaches importance on resolution. On the other hand, a natural image such as a photo or the like preferably undergoes binarization that uses a large matrix size, and attaches importance on tone reproduction.
In this case as well, the multi-valued bitmap image data transferred from the image memory may undergo image area separation to adaptively switch the dither matrix size. However, the same problem as in the above case cannot be prevented.
To avoid such problem, the present applicant has proposed the following method (U.S. patent application Ser. No. 09/434,404). That is, upon rendering a PDL image into a bitmap, attribute information indicating whether the rendered image data corresponds to a natural image area or character/figure area is generated at the same time, and is rendered into a two-dimensional bitmap like the bitmap image. Upon transferring the rendered image data to the image forming unit, the attribute information is transferred at the same time, and different image processes are done on the basis of the attribute information to output a hard copy.
In this case, different dither matrices or different color processes can be applied in correspondence with the aforementioned image attributes. However, when partial images having different attributes are rendered on an identical coordinate position (for example, characters are overwritten on a natural image, or a photo is pasted on a graphic figure), attribute setups of those portions may suffer certain conflicts.
Such conflicts take place when an achromatic gray graphic figure is overwritten on a color natural image and the natural image as the background is set to be seen through the interior of the figure. At this time, since the figure to be overwritten is achromatic, it should be an area to be reproduced using black alone and, for this reason, attribute information indicating a monochrome object is appended to that graphic area. However, since that figure is overwritten so that the background color image can be seen through, if it is reproduced using black alone, color information of the natural image to be seen through in the background is lost.
The two-dimensional map of the attribute information described above also suffers the following problem.
When the object to be processed is only a natural image such as the image 114 shown in FIG. 2A, such method suffices for use. However, when a character (TEXT) image is included in the natural image like an image 7b shown in FIG. 2B, since an attribute “natural image” is determined, a process that is not suitable for the property of the character (TEXT) image is highly likely to be done.
In order to solve such problem, the rasterizer 105 renders R, G, and B signal values into a bitmap image without converting them into C, M, Y, and K signals, and the image forming unit 108 detects a character image area from the received RGB bitmap image using known image area separation, and uses different RGB to CMYK conversion methods for the detected character image area and other areas to generate and output CMYK data.
However, when the entire image undergoes the image area separation, it is time-consuming, and even a pixel of a character may be erroneously determined not to be a character depending on the precision of image area separation.
Especially, when a CPU processes an image area separation program without any hardware, software processes require a longer time.